This is an application to continue longitudinal studies in macaques that have been used to explore mechanisms of safety of a SHIV vaccine, and the remarkable effectiveness of this vaccine not only in conferring long term protection against disease caused by pathogenic SIV and SHIVs, but also elimination of these pathogens given as challenge. Earlier studies established that the vaccine is safe for at least 4 years after inoculation into newborns and after serial passage in young macaques. Studies on efficacy showed that following transient replication, virus could not be re-isolated from the animals but viral DNA persisted for more than 4 years in lymphoid tissues. Immunized animals challenged with pathogenic viruses between 7 and 18 months later became infected but replication of these agents was dramatically curtailed, and in many animals, both the challenge viruses and their DNA became progressively undetectable during three years of post-challenge study. Infectious DNA of the vaccine virus is effective in causing the immunizing infection, and because it induces immune responses to some HIV proteins, it has been proposed to the FDA as a therapeutic vaccine for HIV-infected persons. Data from studies proposed in this application will be used as preclinical support for an IND application to the FDA. New proposed studies will address the following devil's advocate hypothesis: 1. Challenge viruses that are thought to have been eliminated are still present in lymphoid tissues and can be induced by treatment of the challenged animals with anti-CD8+T cell mAb. 2. The vaccine-induced long-term efficacy that correlates with persistence of memory antiviral CD8+T cells, is maintained by antigen provided by persistently-replicating vaccine virus in lymphoid tissues. This will be tested in studies in which some immunized animals will be treated with anti CD8+T cell mAbs to induce the virus. Others will be treated with PMPA to suppress putative virus replication so that immune responses will decay in the absence of antigen. Further, new animals will be inoculated with lymph node cells from virus-negative, immune animals and tested for infection. 3. Pathogenicity of the vaccine virus could be demonstrated by further serial passage of the virus in new animals, and by injection of the agent into macaques that are immunosuppressed by treatment with anti-CD8+T cell mAbs.